Method for the heat treatment of substrates

ABSTRACT

A substrate undergoes a semiconductor fabrication process at different temperatures in a reactor without changing the temperature of the reactor. The substrate is held suspended by flowing gas between two heated surfaces of the reactor. Moving the two heated surfaces in close proximity with the substrate for a particular time duration heats the substrate to a desired temperature. The desired temperature is then maintained by distancing the heated surfaces from the substrate and holding the heated surface at the increased distance to minimize further substrate heating.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/700,298, filed Oct. 31, 2003.

In addition, this application is related to U.S. application Ser. No.10/151,207, METHOD AND DEVICE FOR THE HEAT TREATMENT OF SUBSTRATES,filed May 16, 2002; U.S. application Ser. No. 10/186,269, METHOD ANDAPPARATUS FOR THE TREATMENT OF SUBSTRATES, filed Jun. 27, 2002; U.S.application Ser. No. 10/141,517, TEMPERATURE CONTROL FOR SINGLESUBSTRATE SEMICONDUCTOR PROCESSING REACTOR, filed May 8, 2002; and U.S.application Ser. No. 10/410,699, TEMPERATURE CONTROL FOR SINGLESUBSTRATE SEMICONDUCTOR PROCESSING REACTOR, filed Apr. 8, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor processing and, moreparticularly, to the heat treatment of substrates, includingsemiconductor wafers or flat panel displays.

2. Description of the Related Art

Reactors which can process a substrate while suspending or floating thesubstrate without directly mechanically contacting the substrate, e.g.,by floating the substrate on gas cushions, have relatively recently beendeveloped for semiconductor processing. These reactors may be calledfloating substrate reactors and such a reactor is commercially availableunder the trade name Levitor® from ASM International, N.V. of Bilthoven,The Netherlands.

In the Levitor® reactor, which is also described in U.S. Pat. No.6,183,565 B1, a substrate, such as a wafer, is supported by two oppositegas flows emanating from two heated and relatively massive reactorblocks located on opposite sides of the substrate. The boundary surfacesof the reactor blocks facing the wafer are substantially flat and asmall gap of less than about 1 mm is typically maintained between eachblock and the corresponding wafer surface. The small gap results in aparticularly rapid heat transfer from the heated blocks to the wafer byconduction through the gas. The heat-up of the wafer is thus veryuniform, as the wafer is not mechanically contacted during the heattreatment. In comparison, where a transport arm transports a substrateinto the reactor and then continues to support the substrate duringprocessing, mechanical contact during processing by support fingers of atransport arm results in cold spots on the wafer during heat-up at thecontact positions with the support fingers, as the support fingersrepresent extra thermal mass that needs to be heated and that locallyslows down the heat-up rate. Alternatively, where a substrate istransported to the reactor and then handed off to support pins thatremain in the reactor after processing, mechanical contact duringprocessing by those support pins results in hot spots on the wafer atthe contact positions when the wafer is handed-off and contacts thesupport pins. Also, by floating a substrate during processing, thermalstresses, possibly resulting in crystallographic slip, areadvantageously avoided.

A method utilizing a floating substrate reactor, such as the Levitor®reactor, to achieve a high degree of reproducibility in the thermaltreatment for a series of substrates, which are successively treated oneby one, is described in U.S. Patent Application Publication No.2003/0027094 A1, published Feb. 6, 2003, and assigned to ASMInternational, N.V. In that method, the temperature is measured close tothe boundary surface of a reactor block so that withdrawal of heat fromthe reactor block by the placement of a relatively cold substrate in thereactor is measured at that boundary surface. The reactor block istypically continuously heated and the cold substrate is placed in thevicinity of the reactor block only after the reactor block has reached adesired temperature, as measured at the boundary surface. The coldsubstrate typically will absorb heat and reduce the temperature of thereactor block. The substrate is then removed after processing and beforethe temperature of the continuously heated reactor block rises to thedesired temperature again. After the temperature of the reactor blockrises to the desired temperature, another substrate is placed in thevicinity of the reactor block.

An advantage of reactors such as the Levitor® reactor is that therelatively massive reactor blocks of the reactor act as thermal“fly-wheels,” resulting in a very stable temperature and reproducibleperformance. Ideally, for the most efficient operation, the reactor hasa constant temperature set-point all the time.

Different process requirements, however, may require different treatmenttemperatures. From a semiconductor fabrication operation point of viewand the standpoint of process efficiency, one thermal treatment reactorshould be able to perform these different processes. However, changingthe temperature of reactors such as the Levitor® reactor, andcooling-down the reactor, in particular, is a very time-consumingprocess that can negatively influence the applicability of such areactor for performing sequences of processes requiring differentprocess temperatures.

Accordingly, it is an object of the present invention to provide animproved method for thermally treating a substrate in a floatingsubstrate reactor at different temperatures.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a method is provided for thethermal treatment of a planar substrate. The method comprises providinga reactor having one or more furnace bodies, the one or more furnacebodies each having a substantially flat boundary surface. The one ormore furnace bodies are heated to a predetermined furnace bodytemperature. The substrate is placed adjacent to and essentiallyparallel to the one or more furnace bodies such that a planar surface ofthe substrate faces the boundary surface of each of the one or morefurnace bodies. The substrate is kept adjacent to the boundary surfaceof each of the one or more furnace bodies during a heat-up time to allowthe substrate to heat up to a substrate temperature, wherein thesubstrate temperature is less than the furnace body temperature by about20° C. or more. Subsequently, the substrate is removed from thesubstrate from the reactor while the substrate temperature is still lessthan the furnace body temperature of each of the one or more furnacebodies by about 20° C. or more.

According to another aspect of the invention, a method is provided forthermally treating a substrate. The method comprises providing a firstheated surface at a first temperature and a second heated surface at asecond temperature, where the first heated surface is positioned facingthe second heated surface. A substrate is also provided between thefirst and the second heated surfaces. The substrate is heated to adesired substrate temperature, which is less than the first and thesecond temperatures. The transference of heat between the substrate andthe first and the second surfaces is reduced after heating the substrateto the desired substrate temperature. The reduction in heat transferenceoccurs without reducing the set-point temperature for the first surfaceor the set-point temperature for the second surface. After reducing thetransference of heat, the substrate is maintained between the first andthe second heated surfaces to perform a semiconductor fabricationprocess.

According to yet another aspect of the invention, a method is providedfor semiconductor processing. The method comprises conductively heatinga first thermal treatment substrate in a reactor to a first thermaltreatment temperature by positioning the first thermal treatmentsubstrate in close proximity to a heated reactor surface. The firstthermal treatment temperature is less than the temperature of the heatedsurface. The first thermal treatment substrate is then substantiallymaintained at the first thermal treatment temperature for a firstholding period in the reactor. The method further comprises conductivelyheating a second thermal treatment substrate in the reactor to a secondtemperature higher than the first thermal treatment temperature bypositioning the first thermal treatment substrate in close proximity tothe heated reactor surface. The second thermal treatment substrate isthen substantially maintained at the second temperature for a secondholding period in the reactor. The reactor itself is configured toconductively heat only one substrate at a time. It will be understoodthat the second treatment can precede or succeed the first treatment.

According to another aspect of the invention, a heat treatment apparatusfor processing a plurality of substrates. The apparatus comprises twofurnace bodies which are opposite each other and separated by aseparation distance. Each furnace body has a boundary surface orientedto face a substrate upon positioning of the substrate in the heattreatment apparatus for heat treatment. The furnace bodies are movablerelative to each other. Each furnace body also has a furnace bodytemperature. The apparatus also comprises one or more heaters configuredto heat each furnace body to its furnace body temperature. The apparatusis configured to be able to treat a substrate at either a firsttreatment temperature or a second treatment temperature, where theheater has a substantially constant set-point during treatment whetherthe substrate is at the first treatment temperature or the secondtreatment temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the detailed description ofthe preferred embodiments and from the appended drawings, which aremeant to illustrate and not to limit the invention, and wherein:

FIG. 1 shows, diagrammatically, the loading of a substrate into anexemplary floating substrate reactor, with the reactor shown in an openposition, in accordance with preferred embodiments of the invention;

FIG. 2 shows, diagrammatically, the treatment of a substrate in anexemplary floating substrate reactor, with the reactor shown in a closedposition, in accordance with preferred embodiments of the invention;

FIG. 3 shows, diagrammatically, the treatment of a substrate in anexemplary floating substrate reactor, with the reactor shown in an openposition, in accordance with preferred embodiments of the invention;

FIG. 4 shows the calculated heat-up of a substrate placed symmetricallybetween two reactor bodies maintained at 250° C., at various reactorbody-to-substrate spacings;

FIG. 5 shows the calculated heat-up of a substrate placed symmetricallybetween two reactor bodies maintained at 450° C., at various reactorbody-to-substrate spacings;

FIG. 6 shows the variations over time in thermal treatment temperaturefor a reactor body temperature of 250° C., under various conditions,including differing spacings for differing lengths of time;

FIG. 7 shows the variations over time in thermal treatment temperaturefor a reactor body temperature of 450° C., under various conditions,including differing spacings for differing lengths of time;

FIG. 8 shows, schematically, the sequence of events in an exemplarythermal treatment process, in accordance with preferred embodiments ofthe invention;

FIG. 9 shows, schematically, an exemplary floating substrate reactor inwhich parts of an arm for transporting a substrate remain in the reactorduring processing;

FIG. 10 shows a profile of wafer temperature over time, wherein nitrogengas is flowed between reactor bodies and the wafer;

FIG. 11 shows a profile of wafer temperature over time, wherein heliumgas is flowed between reactor bodies and the wafer; and

FIG. 12 shows a profile of wafer temperature over time, wherein the gasflowed between reactor bodies and the wafer is initially nitrogen gas,which is then switched to helium gas at t=t₁.

DETAILED DESCRIPTION OF THE INVENTION

According to preferred embodiments of the invention, a method isprovided for processing a substrate at different temperatures withoutneeding to change the temperature set-point of the reactor in whichprocessing occurs. Rather than altering the temperature of the reactor,the temperature of the substrate is controlled by varying the amount ofheat energy received by the substrate. In the illustrated embodiments,the substrate is positioned in close proximity with at least one heatedbody or surface of the reactor and heating to a desired temperature isaccomplished by, e.g., holding the substrate in close proximity with theheated surface for a predetermined time. Preferably, there are twoheated bodies, also referred to as furnace bodies, facing each other andthe substrate is positioned between the two heated bodies. Heatingpreferably principally occurs by heat conducted from the heated bodiesto the substrate through a process gas and to a lesser extent byradiation. After being heated to the desired temperature, the heatenergy received by the substrate is decreased to minimize furtherheating of the substrate and, preferably, to prevent the substrate fromsignificantly deviating from the desired temperature.

In one preferred exemplary embodiment, the decrease in thermalconduction occurs by increasing the distance between the substrate andthe heated bodies. In another preferred exemplary embodiment, thedecrease in thermal conduction is brought about by decreasing thethermal conductivity of the process gas, with or without increasing thedistance between the substrate and the heated bodies. In someembodiments, the temperature of the heated bodies and the increaseddistance and/or the thermal conductivity of the process gas are chosensuch that the temperature of the substrate is relatively stable when theheat energy transferred to the substrate is decreased. Advantageously,the decrease in thermal conductivity allows the substrate to undergo asemiconductor fabrication process, such as an anneal, at a reducedtemperature relative to the furnace bodies during the time when thesubstrate is positioned between the furnace bodies.

Preferably, processing of the substrate at the decreased temperature isjust one of two or more processes or process steps to be performed inthe reactor. In such embodiments, the temperature of the heated bodiesis preferably set at the temperature of the highest temperature process,e.g., an anneal. The temperature of the lower temperature processes canpreferably be achieved by use of low thermal conduction between thesubstrate and the heated bodies for a hold time after the initialheat-up time, while the temperature of the higher temperature processcan be achieved by maintaining high thermal conduction between thesubstrate and the heated bodies during that entire process step, e.g.,by having a relatively small separation between the substrate and theheated bodies or by flowing a relatively high thermal conductivity gasbetween the substrate and the heated bodies. Preferably, the substrateis heated to about the temperature of the heated bodies during thehighest temperature process. Thus, by varying the heat transferred tothe substrate, the substrate can be processed at varying temperatureswithout requiring that the temperature of the reactor itself be changed,providing flexibility to use the tool.

Reference will now be made to the Figures, wherein like numerals referto like parts throughout.

While the preferred embodiments can be applied to other reactors knownto those of skill in the art, use of a floating substrate reactor isparticularly advantageous. For example, the reactor design illustratedin FIGS. 1-3 does not require that a substrate 21 be mechanicallysupported during processing; that is, the substrate 21 can be processedwithout being directly contacted by a solid support. This enables veryuniform and rapid heating of the substrate 21 without the cold spotsthat can occur in reactors where substrates are mechanically contactedduring a semiconductor fabrication process. In addition, the upper andlower blocks 13 and 14 of the reactor 1 surrounding the substrate arepreferably relatively massive such that each has a high heat capacityrelative to the substrate 21, helping to stabilize the temperature ofthe substrate and minimizing the susceptibility of the reactor 1 totemperature fluctuations upon loading and unloading of the substrate 21into the reactor 1. The basic configuration of the reactor 1 isavailable commercially under the trade name Levitor® from ASMInternational, N.V. of The Netherlands.

As shown in FIGS. 1 to 3, the heat treating apparatus of the reactor 1includes an upper block 13 and a lower block 14 that are in a housing23. As shown in FIG. 1, the housing 23 is preferably provided with aflap 22 that can be opened for loading and subsequently removing asubstrate 21. The lower block 14 and the upper block 13 can be movedtowards one another by lifting rods 27 and 28. Alternatively, only oneof the blocks is moveable.

The upper block 13 is made up of a upper furnace body 130, an insulatingjacket 131, a heating coil 132 arranged on the inside of the insulatingjacket 131, and an outer jacket 133. Similarly, the lower block 14 ismade up of a lower furnace body 140, an insulating jacket 141, a heatingcoil 142 arranged on the inside of the insulating jacket 141, and anouter jacket 143. Preferably, each furnace body 130, 140 has a massgreater than 10 times the mass of a substrate for which the reactor isdesigned to accommodate, more preferably greater than 40 times thesubstrate mass.

The upper furnace body 130 is preferably provided with at least onetemperature sensor 134 and the lower furnace body 140 is preferably alsoprovided with at least one temperature sensor 144. As described above,in one preferred arrangement, the temperature sensors 134, 144 arearranged to measure temperatures close to the surfaces 146 and 148 ofthe furnace bodies 130, 140 that are adjacent to the substrate 21.

In another arrangement, the upper furnace body 130 is also provided witha second temperature sensor 135 that is arranged close to the side 147of the upper furnace body 130 that faces away from the substrate 21. Ina similar manner, the lower furnace body 140 can be provided with asecond temperature sensor 145 arranged close to the side 149 of thelower furnace body 140 that faces away from the substrate 21. Processgases (including inert gases) are supplied both from the upper furnacebody 130 through openings 25 and the lower furnace body 140 throughopenings 24. The gases can be discharged through a discharge opening 26formed in the reactor housing 23.

The upper block 13 and the lower block 14 are preferably moved apartbefore introducing the substrate 21 into the reactor 1, as shown inFIG. 1. After the substrate 21 has been introduced into the reactor 1,the blocks 13 and 14 are moved towards one another by lifting rods 27and 28 in such a way that the distance between the substrate and theadjacent surfaces 146 and 148 of the furnace bodies 130 and 140,respectively, is preferably less than about 2 mm and more preferablyless than about 1 mm. In the illustrated embodiment, as shown in FIG. 2,the substrate is held in a stable position by gas streams issuing fromthe openings 24 and 25, without requiring further mechanical support. Itwill be appreciated, however, that in other arrangements, supportstructures such as support pins can be used to support and space thesubstrate from the bodies 130 and 140. Moreover, while illustrated asbeing symmetrically spaced from the bodies 130 and 140, the substrate 21can be spaced closer to one or the other of the bodies 130 or 140 inother arrangements.

With reference to FIG. 2, the substrate 21 is heated rapidly after theblocks 13 and 14 are moved towards one another to a closed position,with the substrate 21 accommodated between them. According to onepreferred embodiment, the substrate 21 and the blocks 13 and 14 are heldin these relative positions for only a short heat-up time. Preferably,during the heat-up time, the substrate 21 is only partially heated-up;that is, the heat-up time is not long enough for the substrate 21 beheated to the temperature of the furnace bodies 130 and 140, so that, atthe end of the heat-up time, the temperature of the substrate 21 isstill lower and, preferably substantially lower (e.g., by greater thanabout 10° C.), than the temperature of the furnace bodies 130 and 140.It will be appreciated that the furnace bodies 130 and 140 can be atdifferent temperatures, in which case the temperature of the substrate21 is preferably lower than the temperatures of both the bodies 130 and140. More preferably, the temperatures of the bodies 130 and 140 areapproximately equal. After the heat-up time, the temperature of thesubstrate 21 is preferably less than the higher of the temperatures ofthe bodies 130 and 140 by about 20° C. or more, more preferably, byabout 50° C. or more and, most preferably, by about 100° C. or more.Advantageously, by heating the substrate 21 to these levels or to evenlower temperatures than those indicated by these levels, substrateheating can be more rapid than if the block(s) were set at the desiredsubstrate temperate and the temperature of the substrate 21 can beadequately maintained when the thermal conduction between the substrate21 and the bodies 130 and 140 is decreased, as discussed below.

As shown in FIG. 3, after the heat-up time, the blocks 13 and 14 arepreferably moved away from each other and the distance between thesubstrate 21 and the adjacent surfaces 146 and 148 of the furnace bodies130 and 140, respectively, increases again. After moving the blocks 13and 14 away from each other, the substrate 21 can immediately betransferred out from between the blocks 13 and 14 to a cool-downposition for cooling. Alternatively, the substrate 21 can be heldbetween the blocks 13 and 14 while they are in the moved apart or openposition for an additional hold time before transferring the substrate21 out from between the blocks 13 and 14 to a cool-down position. Duringthis hold time the substrate 21 may further heat up slightly. However,heat transfer by conduction is inversely proportional to the length ofthe heat conduction path. By increasing the distance between thesubstrate 21 and the furnace bodies 130 and 140, the heat transferbetween the substrate 21 and furnace bodies 130 and 140 decreases.

The effect of terminating the heat-up of a substrate, in particular, awafer, before the wafer reaches the temperature of the heated furnacebodies in a reactor such as that shown in FIGS. 1-3 has been calculatedand the results are shown in FIGS. 4 to 7. In particular, FIG. 4 showsthe results of a simulation of the heat-up of a silicon wafer when it isplaced in a floating substrate reactor. In particular, the wafer isplaced between and parallel to two furnace bodies having planar boundarysurfaces facing opposite planar surfaces of the wafer. Heat-up curveswere simulated for a wafer placed equidistantly between the two furnacebodies at a number of different distances, ranging from about 0.15 to 9mm.

In the simulation, heat transfer by both radiation and conductionthrough the gas present between the wafer and the furnace bodies istaken into account. The gas in the simulation is nitrogen at atmosphericpressure. Heat transfer by conduction from the furnace body to the waferis proportional to the temperature difference between the furnace bodyand the wafer and inversely proportional to the length of the conductionpath, i.e., the distance between the wafer and the furnace body. Hence,for small distances the heat transfer by conduction is very high. Asheat transfer by radiation is proportional to the fourth power of thetemperature, at relatively low furnace body temperatures, e.g., below600° C., the heat transfer by radiation is relatively low.

At t=0 the distance between the wafer and each of the furnace bodies isabout 9 mm for all simulations. From t=0 to t=1 s the distance betweenthe wafer and the furnace bodies is reduced, if applicable, by movingthe furnace bodies toward each other. It can be seen from FIG. 4 that,for the larger distances, even after 120 seconds the substratetemperature still significantly deviates from the furnace bodytemperature of 250° C.

In FIG. 5, the results are shown for a furnace body where thetemperature has been raised to 450° C. In this case, the heat-up forlarge distances is somewhat faster as heat transfer by radiation is nowmore significant due to the higher furnace body temperature.Nevertheless, for larger distances the substrate temperature stillsignificantly deviates from the furnace body temperature of 450° C.after 60 seconds.

In FIG. 6, three different heat treatment procedures are simulated forfurnace bodies (FIGS. 1-3) at temperatures of 250° C. In the first,conventional procedure, indicated by Curve “a,” a wafer is placedbetween t=0 and t=1 s in between and adjacent to the two furnace bodies,at a spacing of about 0.15 mm to each one of the furnace bodies. Thenthe wafer is kept in this position for 13 seconds, from t=1 to t=14 s.In the next second, from t=14 to t=15 s, the distance between the waferand each of the furnace bodies is increased from about 0.15 mm to 9 mmby moving each one of the furnace bodies away from the wafer.Subsequently, from t=15 to t=18 s, the wafer is removed from between thefurnace bodies and placed adjacent to a cooling body outside theillustrated reactor 1 (FIGS. 1-3). It can be observed from FIG. 6 thatat t=14 s, the wafer has nearly assumed the temperature (within about 2°C.) of the furnace body.

In the second procedure, indicated by Curve “b,” processing is performedaccording to the preferred embodiments. During the heat-up time, thewafer is spaced about 0.15 mm from the furnace bodies for only about 3seconds, from t=1 to t=4 s. During these 3 seconds, the wafer heats upto about 180° C. Then, in the next second, from about t=4 to t=5 s, thedistance between the wafer and each of the furnace bodies is increasedto about 9 mm. The wafer and furnace bodies are held in this positionfor a holding time of 10 seconds, from about t=5 to t=15 s. During thisadditional holding time, wafer heat-up continues but is substantiallyslower than before and the final wafer temperature increases about 15°C. to about 195° C. Finally, the wafer is transported to the coolingbody from about t=15 to t=18 s.

In the third procedure, indicated by Curve “c,” processing is alsoperformed according to the preferred embodiments. During the heat-uptime, the wafer is also spaced about 0.15 mm from each of the furnacebodies. In this case, however, the heat-up has been reduced to less than3 second, to about 1.5 seconds in this case. The holding time isincreased correspondingly to less than about 12 s, to about 11.5 s, sothat the sum of the heat-up time and the holding time is the same asthat of the first and second procedures. For this third procedure,however, the wafer temperature after heat-up is about 135° C. and, atthe end of the holding time, the wafer temperature has increased about25° C., to about 160° C. Thus, by utilizing the preferred embodiments,the heat-up of a wafer is interrupted and a large variation in waferprocess temperatures can be achieved without having to change thetemperature of the furnace bodies or reactor.

As shown in FIG. 7, similar temperature simulations were carried out forcases where the temperature of the furnace bodies was 450° C. As in FIG.6, Curve “a” is conventional procedure treatment procedure (where thewafer reaches a temperature within about 2° C. of the temperature of theheated bodies) and Curves “b” and “c” are treatment procedures performedaccording to the preferred embodiments. Again, the substrate is spacedabout 0.15 mm from the nearest furnace body during the heat-up periodand about 9 mm from the nearest furnace body during the holding period.Because of the higher initial temperature difference between the waferand the furnace body, relative to the simulation represented by FIG. 6,the initial heat-up part of Curves “a,” “b,” and “c” is steeper than thecorresponding curves of FIG. 6. Consequently, the heat-up times areadvantageously reduced in order to achieve a substantial variation inwafer temperature. For example, the heat-up times have been reduced to 1and 2 seconds for Curves “b” and “c,” respectively.

In addition, during the holding times, about 9 and 10 seconds,represented by Curves “b” and “c,” respectively, a relatively greaterheat-up of the wafer occurs in comparison to the cases represented inFIG. 6. This is because heat transfer through radiation plays anincreasingly larger role at increasingly higher furnace bodytemperatures, as discussed above. In this case, Curve “b” shows a risein temperature from about 300° C. to about 350° C., an increase of about50° C. and Curve “c” shows a rise from about 225° C. to about 300° C.,an increase of about 75° C. Consequently, as furnace body temperatureincreases, the rise in the temperature of the substrate over the targettemperature at the end of the heat-up time also increases. This trendcan be ameliorated somewhat, however, by further increasing the distancebetween the substrate and the furnace bodies, to further decreaseconductive heating of the substrate.

It will be appreciated that FIGS. 5 and 6 illustrate exemplary heatingcurves using exemplary temperatures, substrate to furnace body spacings,heat-up times and hold times. As such, other combinations oftemperatures, substrate to furnace body spacings, heat-up times and holdtimes are possible. For example, the substrate can be spaced less thanabout 0.15 mm from the furnace bodies during the heat-up time or morethan about 9 mm from the furnace bodies during the hold time.Preferably, however, the substrate is spaced less than about 2 mm and,more preferably, less than about 1 mm from the furnace bodies during theheat-up time. In addition, the substrate is preferably spaced more thanabout 2 mm and, more preferably, more than about 4 mm during the holdtime. Moreover, the relatively high temperature of the heated bodiesadvantageously allow rapid heating of the substrate, such that thesubstrate can be heated to a desired temperature in less than about 3seconds, while the reduced transfer of heat between the substrate andthe heated bodies allows for processing at roughly the desiredtemperature, e.g., for about 5 seconds or more and, more preferably,about 8 seconds or more.

A sequence of events in an exemplary embodiment of the invention isshown schematically in FIG. 8, steps a) through f). In step a), thesubstrate 21 is loaded in between upper block 13 and lower block 14, theloading taking a time t_(load). During loading, blocks 13 and 14 arepreferably in an open position for receiving the substrate. In step b),the blocks 13 and 14 are moved towards each other from the open positionto a closed position, the movement to the closed position taking a timet_(close). Then the blocks remain in the closed position during a timet_(heat-up), during which rapid heat-up of the substrate occurs, asshown in step c). According to the preferred embodiments, the heat-uptime t_(heat-up) should be selected to be short enough such that at theend of that time the substrate temperature is still substantially lower,e.g., by less than about 10° C., than the block temperature, asdiscussed above. After the heat-up time has elapsed, the blocks aremoved away from each other to the open position again, taking a timet_(open), as shown in 8 d). The substrate is kept in this position for atime t_(hold), as shown in step e). Finally, the substrate is removedfrom between the blocks, the removal taking a time t_(unload), as shownin step 8 f).

It will be appreciated that any of various methods known in the art canbe utilized to transport a substrate into the reactor 1. For example,the substrate can be transported by a support arm that places thesubstrate between the furnace bodies 130, 140 and then retracts frombetween the furnace bodies 130, 140.

In other arrangements, a substrate can be transported between furnacebodies and the support arm, or parts of the support arm can remainbetween the furnace bodies. In such arrangements, the design of thefurnace bodies can be optimized for optimum temperature uniformity overthe substrate.

For example, in one exemplary embodiment, with reference to FIG. 9, aLevitor® reactor having two furnace bodies 910, 920, configured foraccommodating a substrate 930 therebetween is shown schematically.Preferably, the furnace bodies 910, 920 are massive and the surfaces ofthe furnace bodies 910, 920 facing the substrate 930 are as perfectlyplanar as possible. However, during transport of the substrate 930towards the furnace bodies 910, 920, the substrate 930 is mechanicallysupported by the support fingers 940. During processing, these fingers940 are accommodated into recesses 950 in the lower furnace body 920. Atthe position of the recesses 950, heat transport toward the substrate930 by conduction is locally limited due to the relatively largedistance between the surfaces of the furnace bodies 910, 920 and thesubstrate 930. Advantageously, this can be at least partly compensatedfor by providing elevated areas 910 in the furnace body 910 opposing thefurnace body 920 having the recess 950, the elevated areas 910 locatedat locations corresponding to the recesses 950. Because of the elevatedareas 960, the distance between the substrate 930 and upper furnace body910 locally decreases, resulting in locally increased heat transfer fromthe upper furnace body 910 to the substrate 930. In one example, thedistance between substrate 930 and the main surface of the furnace body910 during processing is about 0.15 mm and the height h of the elevatedarea 960 is between about 0.05 and 0.10 mm. Preferably, the lateralsizes of the elevated areas approximately match with the sizes of theportions of the recesses overlapped by the wafer 930.

EXAMPLE

A silicidation process involving two anneals was carried out. The firstanneal of these two anneals was carried out according to either aconventional anneal or to an anneal according to the methods describedherein and the results for these two anneals were compared.

In the conventional first anneal, silicon wafers having a nickel (Ni)film on their top surface received an anneal in a Levitor® reactor,commercially available from ASM International, N.V. of Bilthoven, TheNetherlands, with the reactor blocks and furnace bodies at a constanttemperature of 350° C. The blocks remained in the closed position, withthe wafer at a distance of 0.15 mm from each of the reactor blocks, for18 seconds.

In the first anneal according to the methods described herein, similarsilicon wafers received an anneal in the Levitor® reactor with thereactor blocks and furnace bodies set at a temperature of 450° C. Afterintroducing the wafer into the reactor, the blocks remained in theclosed position for 2.6 seconds, which, according to the above describedtemperature simulations, should give a wafer temperature of about 340°C. Then the blocks were moved away from each other so that the distancebetween the wafer and each of the blocks was 9 mm and the wafer was keptin this condition for a holding time of 5 seconds.

During the first anneal of the wafers Ni₂Si is formed in those areaswhere the Ni is in contact with silicon. After this step, in a typicalIntegrated Circuit fabrication fabrication process, unreacted Nickel isselectively removed from the wafer by an etching process, leaving apattern of Ni₂Si on the wafer in the areas where the silicon was exposedto Ni. After selective removal of unreacted Ni a second anneal iscarried out to form low resistivity NiSi. In this example, a secondanneal was carried out in the Levitor® reactor at 450° C. for 40seconds. It will be appreciated that the second anneal can be carriedout in the same reactor as the first anneal or it can be carried out ina different, but similarly configured, reactor.

After the second anneal, the sheet resistance was 7.5 Ohm/sq. for thewafer that received a conventional first anneal and 7.7 Ohm/sq. for thewafer that received the first anneal according to the methods describedherein. The similarity in these results indicates that the methodsdisclosed herein are an effective substitute for the conventional annealand allow the performance of a thermal treatment where the temperaturespecified for the treatment is less than the temperature set-point ofthe reactor.

It will be appreciated that other combinations of heat-up times and holdtimes can be utilized to arrive at similar results. In addition, asnoted above, rather than using two reactors set at two differenttemperatures, the same reactor can be used for both the first and thesecond anneals, with the reactor blocks and furnace bodies set at thetemperature of the higher temperature anneal. It will also beappreciated that the substrate can be removed from the reactor for otherprocessing, e.g., for film deposition on the substrate or a patterningprocess, in the time between the first and the second anneals, and thesecond anneal can be performed on a physically different wafer.Moreover, in other process sequences, rather than performing two annealson one wafer, the same reactor can be used to process different wafersrequiring different anneal temperatures, so that the first process orprocess step can be carried out on one wafer, while the second processor process step can be carried out on a different wafer undergoing adifferent process sequence. Furthermore, the described reactor iscapable of thermal treatments beyond simple annealing, such as chemicalvapor deposition (CVD).

Although the preferred embodiments have been described with a substratefloatingly supported by gas streams between two furnace bodies duringthe heat-up and hold times, the embodiments of the invention are notlimited to such as arrangement. For example, in other embodiments, thewafer can be mechanically supported during the treatment. Thus, in someembodiments, during the heat-up time, the wafer can be supported on afurnace body or on support structures such as pins and during the holdtime the wafer may be supported spaced from the furnace body on supportstructures also. In addition, the wafer can be heated using only onefurnace body and/or, during the hold time, the distance between thesubstrate and one furnace body can be different from the distance of thesubstrate to the other furnace body.

Moreover, various other arrangements are possible for lowering theamount of heat transferred from the furnace body or bodies. For example,instead of varying the distance between the substrate and the furnacebodies to change the heat transfer between the substrate and the furnacebodies, the distance may be kept constant and the gas between thesubstrate and the furnace bodies can be varied. In some embodiments,during heat-up a high conduction gas, such as He or H₂, can be used andduring the hold time a lower conduction gas, such as N₂ or Ar, can beused. It will be appreciated that the difference in thermal conductionbetween He and N₂ is a factor of about 10. In other embodiments, thedistance between the substrate and the furnace body or bodies can beincreased and the lower conduction gas can be used during the hold timeto reduce heat conduction. Advantageously, such embodiments can furtherreduce the heating that occurs during the hold time and are particularlyadvantageous for minimizing further substrate heating in relatively hightemperature situations such as that represented in FIG. 7.

In addition, it will be appreciated that, while illustrated with twoheated furnace bodies, only one heated furnace body need be provided orheated. In such cases, heating is principally due to heat transfer fromthe single heated body and varying the heat transfer between the singleheated body and the substrate varies the temperature of the substrate.

Also, rather than lowering the heat transfer to a substrate after theheat-up time, in an alternative application, the heat transfer can beincreased after the heat-up time. This can be performed by applying afirst spacing between the substrate, e.g., a wafer, and each of thefurnace bodies during an initial heat-up period, and then a secondspacing after the initial heat-up, wherein the second spacing is smallerthan the first spacing. Alternatively, the heat transfer can be variedby changing the heat conduction of the gas between the wafer and each ofthe furnace bodies, having a gas with a first heat conduction during aninitial heat-up period and a gas with a second heat conduction after theinitial heat-up period, wherein the second heat conduction is greaterthan the first heat conduction.

In FIGS. 10, 11 and 12 this alternative application is explained infurther detail. The results of these figures were obtained bysimulation. FIG. 10 shows an exemplary wafer temperature-time profile inwhich the furnace bodies are at about 1000° C. and nitrogen gas isflowed between the furnace bodies and the wafer. FIG. 11 shows anexemplary wafer temperature-time profile in which helium has is flowedbetween the furnace bodies and the wafer. Helium has a higher thermalconductivity than nitrogen and, consequently, in FIG. 11 a steeperheat-up can be observed than in FIG. 10. However, the heat-up can be sofast that damage to the wafer and/or to the furnace bodies occurs. Thismay be particularly true in the low temperature region since the heattransfer mechanism is conduction, which results in a heat flow that isproportional to the temperature difference between the furnace bodiesand the wafer. Consequently, initially, when the wafer is close to roomtemperature, the temperature difference is large, resulting in a largeheat flow, a fast heat-up and a large thermal shock. The resistance ofthe furnace bodies against thermal shocks depends on the material of thefurnace bodies: furnace bodies of graphite and quartz are better able towithstand thermal shocks than furnace bodies made of silicon carbide.

FIG. 12 shows an exemplary wafer temperature-time profile, in whichnitrogen gas is flowed between the furnace bodies and the wafer duringthe initial heat-up. At t=t₁ the nitrogen gas is replaced by helium,resulting in an increase of the heat-up rate. In most cases, when aspike anneal is required, the upper part of the spike determines thethermal budget and process performance. The part of the profile having atemperature of about 100° C. or more below the peak temperature of thespike does not have a significant influence. According to this method,the wafer can be subjected to a sharp spike anneal at high temperature,while avoiding problems, e.g., large thermal shocks, at lowertemperatures.

Accordingly, it will be appreciated by those skilled in the art thatother various omissions, additions and modifications can be made to theprocesses described above without departing from the scope of theinvention. All such modifications and changes are intended to fallwithin the scope of the invention, as defined by the appended claims.

1. A heat treatment apparatus for processing a substrate, comprising:two furnace bodies disposed opposite each other and separated by aseparation distance, each furnace body having a boundary surfacepositioned to face a substrate upon retention of the substratetherebetween, wherein the furnace bodies are movable relative to eachother; and one or more heaters configured to heat each furnace body,wherein the apparatus is configured to maintain the substrate at atemperature below temperatures of the furnace bodies during an entiretime the substrate is retained between the furnace bodies.
 2. Theapparatus of claim 1, wherein the apparatus is configured to float thesubstrate on a gas cushion between the furnace bodies.
 3. The apparatusof claim 1, wherein the temperature of the substrate is about 20° C. ormore less than the temperatures of the furnace bodies during the timethe substrate is retained between the furnace bodies.
 4. The apparatusof claim 1, wherein the temperatures of the two second furnace bodiesare the same.
 5. The apparatus of claim 1, configured to maintain thesubstrate at the first temperature for a first holding period and tomaintain the substrate at the second temperature for a second holdingperiod.
 6. The apparatus of claim 1, configured to maintain thesubstrate spaced from the furnace bodies at a first set of distancesduring an initial heat up time and to increase the distances between thesubstrate and the furnace bodies during a subsequent holding time. 7.The apparatus of claim 6, configured to space the substrate from thefurnace bodies at less than about 2 mm during the heat up time and atmore than about 2 mm during the holding time.
 8. The apparatus of claim7, configured to space the substrate from the furnace bodies at lessthan about 1 mm during the heat up time.
 9. The apparatus of claim 8,configured to space the substrate from the furnace bodies at more thanabout 4 mm during the holding time.
 10. The apparatus of claim 1, in gascommunication with a source of a low thermal conductivity gas and asource of a high thermal conductivity gas, wherein the apparatus isconfigured to supply a low thermal conductivity gas and a high thermalconductivity gas between the substrate and the furnace bodies.
 11. Theapparatus of claim 10, configured to first supply the high thermalconductivity gas between the substrate and the furnace bodies andsubsequently to supply the low thermal conductivity gas between thesubstrate and the furnace bodies.
 12. The apparatus of claim 1, whereinthe substrate is a semiconductor substrate.
 13. A heat treatmentapparatus for processing a plurality of substrates one at a time,comprising: two furnace bodies, the furnace bodies opposite each otherand separated by a separation distance, each furnace body having aboundary surface oriented to face a substrate upon positioning of thesubstrate in the heat treatment apparatus for heat treatment, whereinthe furnace bodies are movable relative to each other; and heatersassociated with and configured to heat each furnace body duringsubstrate treatment, wherein the apparatus is configured to heat asubstrate to a first substrate treatment temperature or a secondsubstrate treatment temperature, wherein the heaters have asubstantially constant temperature set-point during treatment whetherthe substrate is heated to the first treatment temperature or the secondtreatment temperature.
 14. The apparatus of claim 13, wherein each ofthe furnace bodies is independently movable.
 15. The apparatus of claim13, wherein the one or more heaters are configured to heat the furnacebodies to the same temperature.
 16. The apparatus of claim 13,configured to maintain the first and the second treatment temperaturesat less than the furnace body temperature of each furnace body by about20° C. or more.
 17. The apparatus of claim 13, configured to move thefurnace bodies to each of three positions: a moved apart position forintroducing or removing a substrate therebetween; a closed positionduring a heat-up time after introducing a substrate, wherein thedistance between the substrate and each of the furnace bodies is lessthan 2 mm in the closed position; and a hold position after heat-up ofthe substrate, wherein the distance between the wafer and each of thefurnace bodies is greater in the hold position than in the closedposition.
 18. The apparatus of claim 17, configured to hold the furnacebodies in the closed position for a time period selected based upon: atemperature of each of the furnace bodies; a distance between thesubstrate and each of the furnace bodies in the closed position; and adesired treatment temperature.
 19. The apparatus of claim 18, whereinthe desired treatment temperature is a highest temperature of thesubstrate while retained between the furnace bodies, wherein the timeperiod the furnace bodies are held in the closed position is selected tomaintain the temperature of the substrate below the temperatures of eachof the furnace bodies.
 20. The apparatus of claim 13, configured tofloat the substrate between the furnace bodies on a gas cushion.
 21. Aheat treatment apparatus for processing a plurality of substrates one ata time, comprising: two furnace bodies, the furnace bodies opposite eachother and separated by a separation distance, each furnace body having aboundary surface oriented to face a substrate upon positioning of thesubstrate in the heat treatment apparatus for heat treatment, whereinthe furnace bodies are movable relative to each other; and heatersconfigured to heat each furnace body during an initial substrate heat-upperiod and a subsequent substrate holding time, wherein each furnacebody is provided with an associated heater, wherein the apparatus isconfigured to establish a first heat transfer rate between each of thefurnace bodies and the substrate during the initial heat-up period and asecond heat transfer rate between each of the furnace bodies and thesubstrate during the subsequent holding time, wherein the heater has asubstantially constant temperature set-point during treatment.
 22. Theapparatus of claim 21, wherein the one or more heaters are configured toheat the furnace bodies to the same temperature.
 23. The apparatus ofclaim 21, wherein the second heat transfer rate is lower than the firstheat transfer.
 24. The apparatus of claim 21, wherein the second heattransfer rate is higher than the first heat transfer.
 25. The apparatusof claim 23, configured to maintain the substrate spaced from thefurnace bodies at a first set of distances during the initial heat-upperiod and to increase the distances between the substrate and thefurnace bodies during the subsequent holding time.
 26. The apparatus ofclaim 23, configured to supply a high thermal conductivity gas betweenthe substrate and the furnace bodies during the initial heat-up periodand to supply a low thermal conductivity gas during the subsequentholding time.
 27. The apparatus of claim 24, configured to maintain thesubstrate spaced from the furnace bodies at a first set of distancesduring the initial heat-up period and to decrease the distances betweenthe substrate and the furnace bodies during the subsequent holding time.28. The apparatus of claim 24, configured to supply a low thermalconductivity gas between the substrate and the furnace bodies during theinitial heat-up period and to supply a high thermal conductivity gasbetween the substrate and the furnace bodies during the subsequentholding time.